X-ray lithography, because of its high resolution and excellent process latitude, offers considerable benefits over other lithography methods for producing devices with lateral dimensions in the vicinity of 0.5 micron and below. The membranes that are used in X-ray lithography typically comprise a thin foil substrate supporting an X-ray absorbing pattern. Since the thin foil substrate is usually only a few micrometers thick, it usually requires an additional peripheral supporting structure.
Selection of a suitable material for the X-ray lithography membrane is not a simple process for many reasons. For example, an X-ray lithography membrane should have a usable area as large a possible (ideally as large as a silicon wafer), a thickness in the micron (.mu.m) range, to minimize absorption of X-rays and subsequent rise in membrane temperature, virtually absolute flatness, high strength, dimensional and mechanical stability against radiation, humidity, and heat, and compatibility with X-ray absorber materials such as gold, tantalum and tungsten. The membrane must also have sufficient transparency to X-rays so that adequate contrast can be achieved, and transparency to visible light for alignment purposes.
To meet these requirements, materials such as silicon, boron doped silicon, boron nitride, silicon nitride (Si.sub.3 N.sub.4 and SiN.sub.x), silicon oxide (SiO.sub.2 and SiO.sub.x), beryllium, silicon carbide (SiC and SiC.sub.x), tungsten carbide, silicon oxynitride, alumina, Mylar, and Kapton have been employed as X-ray lithography membranes. Illustrative are the following references: U.S. Pat. Nos. 3,742,230; 3,925,677; 4,037,111; 4,198,263; 4,260,670;.Yamada, et al., Microelectronic Eng'r 9, 135-138 (1989); Mackens, et al., Ion-Beam Technology, Submicrometer Lithographics VII, 9-15 (1988); Suzuki, Electron Beam, X-ray and Ion Beam Lithographics VI, 23-29 (1987), Ku, et al., J. Vac. Sci. TechnoI. B. 6, No. 6, 2174-2177 (1988); Uzoh, et al., J. Vac. Sci. Technol. 6, No.6, 2178-2183 (1988); Aiyer, et al., Thin Solid Films 163, 229-232 (1988).
These disclosed materials meet some of the above mentioned requirements well but only marginally satisfy one or more of the other requirements. For example, membranes made of boron nitride have been noted to lose optical transparency when exposed to X-rays from a synchrotron radiation source. The optical and X-ray transparencies of many of the above described materials have also been noted to decrease with increasing thickness (generally above 1 .mu.m). Further, because of the low thermal conductivity of the current X-ray membrane materials, the temperature of these membranes has been noted to rise with the absorption of X-rays by the absorber material. This rise in temperature coupled with the high coefficient of thermal expansion of these materials results in significant distortion of the pattern and damage to the membrane.
X-ray lithography membranes made out of thin diamond film (a pure crystalline carbon structure) offer a viable solution to many of the problems experienced by the current X-ray lithography membrane materials. Diamond has a high thermal conductivity (.about.2000 W.sup.-1 m.sup.-1 K.sup.-1) which facilitates the rapid dissipation of heat generated by the absorption of X-rays. It also minimizes the pattern distortion due to its low coefficient of thermal expansion (.about.1.1.times.10.sup.-6 /.degree.C.) and high Young's modulus. Additionally, diamond is extremely hard, X-ray and optically transparent, and extremely durable and resistant to chemical attack. Several prior art techniques have been disclosed for producing diamond (or amorphous carbon) membranes. Illustrative are the following references: U.S. Pat. No. 4,436,797; and Japanese, Patent Application Nos. 63-979, 62-17152, and 62-89586.
U.S. Pat. No. 4,436,797 describes a process for fabricating a X-ray membrane from amorphous carbon deposited on silicon wafer by plasma assisted CVD using a mixture of hydrogen and hydrocarbon.
Japanese Patent Application No. 63-979 filed on 5 Jan. 1988 discloses a X-ray lithography membrane made of diamond film. The thickness of diamond film is claimed to be .about.1 .mu.m, and is deposited on silicon wafer by hot filament, high-frequency plasma, or microwave plasma CVD method with CH.sub.4 /H.sub.2 mixture at 800.degree. to 1000.degree. C. and pressure of several torr. This patent application fails to provide details of essential processing parameters, such as composition and flow rate of feed gas and operating pressure, required to deposit thin diamond film either in tension or free of compressive stresses.
Japanese Patent Application No. 62-17152 filed on 29 Jan. 1987 describes the use of diamond thin film as a X-ray lithography membrane. It claims that the use of CVD diamond film as a X-ray membrane has been difficult because of rough surface finish. The surface roughness of CVD diamond has been reduced by polishing, thereby enabling one to use it as X-ray membrane. The diamond film is deposited on silicon by using either microwave plasma CVD or hot-filament CVD. A gaseous feed mixture containing -% CH4 in H2 was used at 900.degree. C. substrate and 2000.degree. C. filament temperatures to deposit diamond by HFCVD. This patent application fails to provide details of processing parameters required to deposit diamond film either in tension or free of compressive stresses.
Japanese Patent Application No. 62-89586 filed on 10 Apr. 1987 discloses a X-ray membrane made of carbon-based film containing diamond crystals. The carbon-based membrane is produced by ionizing a mixture of hydrogen, hydrocarbon, organic compound and inert gas by an ion beam in a vacuum chamber.
A drawback of these prior art techniques is that since the coefficient of thermal expansion of diamond (.about.1.1.times.10.sup.-6 .degree.C..sup.-1) is considerably lower than that of the base silicon material (.about.4.2.times.10.sup.-6 .degree.C..sup.-1), the diamond film is generally deposited with residual compressive stresses. These compressive stresses produce wrinkles in the film when the base material is removed by chemical etching, thereby producing poor quality X-ray membrane.
It is therefore an object of this invention to provide a substantially compressive stress free, pin-holes and defects free, continuous polycrystalline diamond thin membrane for X-ray lithography.
It is a further object of this invention to provide a substantially compressive stress free, substantially optically and X-ray transparent membrane for X-ray lithography.
It is a further object of this invention to provide a low cost and efficient process for producing a substantially compressive stress free, pin-holes and defects free, and substantially optically and X-ray transparent continuous polycrystalline diamond membrane for X-ray lithography.